The present invention relates to a transistor firing circuit of the type including a firing transformer having a primary winding across which there is produced a flyback voltage which is limited in a manner to remain constant within narrow, defined limits over the operating temperature range of the circuit.
Over a wide temperature range, e.g. between -28.degree. C. and +115.degree. C., such transistor firing circuits must have a relatively constant flyback voltage across the primary winding. This constant voltage can be realized with a corresponding number of flux diodes which are connected in series with a Zener diode. Depending on the rated voltage U.sub.Z and the temperature coefficient T.sub.K,Z of the Zener diode employed and the rated voltage and temperature coefficient of the flux diodes, a relatively large number of flux diodes would be required to effect temperature compensation. The following example of the prior art will serve to explain this in detail.
A Zener diode with a rated voltage U.sub.z =150 V and a temperature coefficient T.sub.K,Z =+10.multidot.10.sup.-4 K.sup.-1 is temperature compensated by flux diodes with a rated voltage U.sub.F =0.6 V and having a temperature coefficient T.sub.K,F =-33.multidot.10.sup.-4 K.sup.-1. The change in voltage of the Zener diode is 150.multidot.10.multidot.10.sup.-4 VK.sup.-1 =0.150 VK.sup.-1 ; the change in voltage of a flux diode is -0.6 V.multidot.33.multidot.10.sup.-4 K.sup.-1 =0.002 VK.sup.-1. Consequently, (150:2)=75 flux diodes must be connected in series. However, such a series circuit has a total voltage drop of 150 V+75.0.multidot.0.6 V=195 V.